1. Field of the Invention
This invention relates to a GaAlAs semiconductor laser device, and more particularly, to an improved GaAlAs semiconductor laser device with a quantum well structure.
2. Description of the Prior Art
In recent years, thin single crystal film growth techniques such as molecular beam epitaxy (MBE) and metal organic-chemical vapor deposition (MO-CVD), have been rapidly developed which enables the formation of extremely thin epitaxial growth layers having a thickness of as thin as approximately 10 .ANG.. Due to the progress in these crystal growth techniques, it is possible to make laser devices based on device structures having very thin layers which could not be easily produced by conventional liquid phase epitaxy (LPE). A typical example of these laser devices is a quantum well (QW) laser device, in which the active layer has a thickness of 100 .ANG. or less resulting in the formation of quantum levels therein, whereas the active layer of conventional double-heterostructure (DH) laser devices has a thickness of several hundreds of angstroms or more. Thus, this quantum well laser is advantageous over the conventional double-heterostructure lasers in that the threshold current level is reduced and the transient characteristics are superior. Such quantum well laser devices are described in detail in the following articles:
(1) W. T. Tsang, Applied Physics Letters, vol.39, No. 10, pp.786 (1981); and
(2) H. Iwamura, T. Saku, T. Ishibashi, K. Ohtsuka, Y. Horikoshi, Electronics Letters, vol.19, No.5, pp.180 (1983).
As mentioned above, by the use of thin single crystal film growth techniques such as MBE and MO-CVD, it is now possible to put high-performance semiconductor lasers into practical use.
FIG. 6 shows a GaAlAs graded-index separate confinement heterostructure (GRIN-SCH) semiconductor laser device with a single quantum well structure. The AlAs mole fraction in the active region of this semiconductor laser device is shown in FIG. 7. Reference numerals 44 to 48 given in this figure along the abscissa axis indicating the direction of a layer thickness denote the regions corresponding to respective layers of the laser device shown in FIG. 6, respectively. That is, the double-heterostructure of this laser device is composed of a cladding layer 44, a GRIN optical guiding layer 45, a quantum well active layer 46, a GRIN optical guiding layer 47, and a cladding layer 48, which are disposed in the direction of a layer thickness.
The quantum well structure provides a high radiation efficiency based on the quantum effect thereof. However, the thickness of the quantum well active layer is about 100 .ANG. or less, which is smaller than the wavelength of laser light by one order or more of magnitude, so that the active layer cannot confine the laser light. On the other hand, semiconductor laser devices must give rise to induced emission, and the gains thereof are proportional to the density of photons within the quantum well. Therefore, in the GRIN-SCH structure, laser light is guided through the GRIN optical guiding layers 45 and 47 with a gradient of the AlAs mole fraction, which are superposed on the quantum well active layer 46, respectively, resulting in enhanced efficiency of the interaction between the carriers within the quantum well and the laser light.
However, in the GRIN-SCH semiconductor laser device with a single quantum well structure, the first and second GRIN optical guiding layers 45 and 47 have a thickness of about 1500 to 2000 .ANG., whereas the thickness of the quantum well active layer 46 is quite small to the order of 100 .ANG. or less. Therefore, in the conventional double-heterostructure laser device, 20 to 30% or more of laser light is confined within the active layer, whereas the GRIN-SCH semiconductor laser device with a single quantum well structure has a coefficient .GAMMA. of confinement of laser light within the quantum well active layer of 10 to 5% or less, which is quite small as compared with the double-heterostructure laser device.
As mentioned above, the GRIN-SCH semiconductor laser device with a single quantum well structure has gains enhanced by the quantum effect despite its small coefficient .GAMMA. of confinement, and it is possible to reduce the threshold current density, which is 500 A/cm.sup.2 or more for the conventional double-heterostructure laser device, to 200 A/cm.sup.2 or less. Such a decrease in the threshold current density can also be attained in a SCH semiconductor laser device with a quantum well structure comprising optical guiding layers with a constant AlAs mole fraction.
On the other hand, the double-heterostructure laser device usually has a thin active layer to attain high output power. The improvement in output power by making the active layer thin is disclosed by, for example, K. Hamada et al., IEEE Journal of Quantum Electronics, vol. QE-21, pp.623 (1985) and T. Murakami et al., IEEE Journal of Quantum Electronics, vol. QE-23, pp.712 (1987).
However, when the active layer has a thickness of about 1000 .ANG. or less, the amount of laser light transmitted to the cladding layer increases, resulting in a decrease in the coefficient .GAMMA. of confinement. Therefore, when the thickness of the active layer is about 500 .ANG. or less, as the active layer becomes thinner, the ratio of the recombination region decreases, resulting in an increase in the threshold current. However, because of a decrease in the density of photons within the active layer, it is possible to raise a maximum optical output power level which causes a disruption of the facets of a laser device. Based on such an idea, as semiconductor laser devices with an improved device structure in which the coefficient .GAMMA. of confinement is further reduced and which does not need an increase in the threshold current, there can be mentioned a GRIN-SCH and a SCH semiconductor laser devices with a quantum well structure. These laser devices are used as those for superhigh output power; see, for example, D. R. Scifres et al., Applied Physics Letters, vol.41, pp.1030 (1982) and D. R. Scifres et al., Electronics Letters, vol.19, pp.169 (1983).
The inventors have investigated quantum well laser devices, and found that the upper limit of optical output power of these laser devices is not determined by the coefficient .GAMMA. of confinement.
The GRIN-SCH semiconductor laser devices with a single quantum well structure shown in FIG. 7 was produced as follows: On the (100) plane of a Si-doped n-GaAs substrate (Si=2.times.10.sup.18 cm.sup.-3) 41, a Si-doped n-GaAs buffer layer (Si=1.times.10.sup.18 cm.sup.-3 ; the thickness thereof being 0.5 .mu.m) 42, a Si-doped n-Ga.sub.1-v Al.sub.v As graded buffer layer (Si=1.times.10.sup.18 cm.sup.-3 ; the thickness thereof being 0.2 .mu.m) 43, a Si-doped n-Ga.sub.0.3 Al.sub.0.7 As cladding layer (Si=1.times.10.sup.18 cm.sup.-3 ; the thickness thereof being 1.4 .mu.m) 44, an undoped GaAlAs GRIN layer (the thickness thereof being 0.15 .mu.m) 45, an undoped GaAs quantum well active layer 46, an undoped GaAlAs GRIN layer (the thickness thereof being 0.15 .mu.m) 47, a Be-doped p-Ga.sub.0.3 Al.sub.0.7 As cladding layer (Be32 5.times.10.sup.17 cm.sup.-3 ; the thickness thereof being 1 .mu.m) 48, a Be-doped p-GaAs cap layer (Be =2.times.10.sup.18 cm.sup.-3 ; the thickness thereof being 0.2 .mu.m) 49, a Si-doped n-Ga.sub.0.5 Al.sub.0.5 As current blocking layer (Si=1.times.10.sup.18 cm.sup.-3 ; the thickness thereof being 0.6 .mu.m ) 50, and a Si-doped n-GaAs contact layer (Si=1.times.10.sup.18 cm.sup.-3 ; the thickness thereof being 0.2 .mu.m) 51 were successively grown by molecular beam epitaxy. The temperature of the substrate was set to be 720.degree. C. during the growth and the flux ratio of group V to group III was about 2.5.
After the growth, the central portion of each of the contact layer 51 and the current blocking layer 50 was selectively removed into a striped shape having a width of 100 .mu.m by an etching technique using an H.sub.2 SO.sub.4 etchant and an HF etchant. Then, an n-sided electrode 52 of AuGe/Ni/Au and a p-sided electrode 53 of AuZn/Au were disposed on the back face of the substrate 41 and the upper face of the contact layer 51 including the cap layer 49 and the current blocking layer 50, respectively, by the vacuum evaporation method.
The AlAs mole fraction v in the n-Ga.sub.1-v Al.sub.v As graded buffer layer 43 was changed in the range of 0.1 to 0.7 according to the linear distribution. Moreover, the AlAs mole fractions in the undoped GaAlAs GRIN layer 45 and 47 were changed in the range of 0.7 to 0.2 and in the range of 0.2 to 0.7, respectively, according to the parabolic distribution.
The thickness Lz of the quantum well active layer 46 was set to different values in the range of 40 to 300 .ANG., and various semiconductor laser devices with different thickness Lz were produced in the same manner as mentioned above.
The wafer obtained was then cleaved to form a laser device unit with a cavity length of 375 .mu.m. The facets on both sides of the laser device were coated with an Al.sub.2 O.sub.3 film and a multi-layered film made of Al.sub.2 O.sub.3 and Si, respectively, by the electron beam deposition method, so that the reflective indices of these coated facets were about 5 and 90%, respectively. The unit was then mounted on a copper heat sink by means of a soldering material such as In, resulting in a semiconductor laser device of FIG. 6.
The resulting semiconductor laser device oscillated a laser beam at a threshold current of 150 to 300 mA when driven with a direct current. At that time, the optical output power level of a laser beam emitted from the front facet having a reflective index of 5% was monitored, and it was found that the disruptive power level is almost constant at 1.2 .+-.0.1 W when the thickness Lz of the active layer 46 is in the range of 40 to 200 .ANG., but decreases slightly to 1 W or less when the thickness Lz equals to 300 .ANG..
The coefficient .GAMMA. of confinement of laser light within the quantum well of these laser devices is nearly given by (Lz/3000).times.100% (where the thickness Lz is in .ANG.). Therefore, even when the thickness Lz is in the range of 40 to 200 .ANG. at which the upper limit of the disruptive power level is almost constant, the coefficient .GAMMA. of confinement varies greatly in the range of 1.3 to 7%. In this way, it was found that because the GRIN-SCH semiconductor laser device with a quantum well structure has the coefficient .GAMMA. of confinement quite smaller than that of the conventional double-heterostructure laser device, the upper limit of optical output power of the GRIN-SCH semiconductor laser device is not determined by the coefficient .GAMMA. of confinement. This fact also holds in the case of the SCH semiconductor laser device with a quantum well structure which has a coefficient .GAMMA. of confinement of 10%.